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ananyo writes "Black cottonwood trees (Populus trichocarpa) can clone themselves to produce offspring that are connected to their parents by the same root system. Now, after the first genome-wide analysis of a tree, it turns out that the connected clones have many genetic differences, even between tissues from the top and bottom of a single tree. 'When people study plants, they'll often take a cutting from a leaf and assume that it is representative of the plant's genome,' says Brett Olds, a biologist at the University of Illinois at Urbana-Champaign who was involved in the study. 'That may not be the case. You may need to take multiple tissues.' The finding also challenges the idea that evolution only happens in a population rather than at an individual level. As one tree contains many different genomes, natural selection and evolution could happen within a single organism."

Some marmosets [pnas.org] are naturally chimeric some substantial portion of the time. This leads to wacky fun for researchers because it is perfectly possible(depending on how the different cell populations ended up distributed in the mature monkey) for an individual to show one genotype on blood tests; but produce offspring that appear to be genetic descendants of their brother or sister....

So called Clonally transmissible cancers [wikipedia.org] are particularly growth-oriented cells from some progenitor organism that managed to beat the odds and, instead of just killing their luckless host as cancers tend to, spread to other members of the species.

There is also Henrietta Lacks; but she lives more or less exclusively in laboratory environments and might not be said to count...

And if anyone's wondering, it happens in humans too. One woman [wikipedia.org] nearly had her kids taken away when DNA tests indicated she wasn't their mother, until it was determined that her reproductive system was from one of her constituent maternal "twins" while her hair and skin (which were sampled for the tests) were from another.

Trees do not have "active" immune systems like animals, that cause "transplant rejection". The tree needs leaves and it needs roots, but as long as the leaves do reasonably "leafy" stuff they could be genetically different and the rest of the tree will go on fine. That's why you can often graft the top of one tree species onto the bottom part of another tree.

In contrast it's not trivial to put a related human's kidney into another human. You'd likely still have to suppress the immune system.

It could be because a tree doesn't need as much per in terms of resources (energy etc) per mass/volume, and it doesn't need to move. So some inefficiencies due to "cancer" (strange growths) are less likely to kill the tree. Thus it does not need to kill cancer as urgently.

Whereas strange growths are likely to kill you - once they are large enough so you can't move about, feed or breathe you're going to die.